The fourth generation of telecommunication networks (4G) encompasses Multi-User Multiple-Input Multiple Output (MU-MIMO) systems. A MU-MIMO system allows multiple users to communicate using a plurality of transmission antennas both on each user equipment and each network transmission unit (e.g. Base Station, eNodeB etc. . . . ). A MU-MIMO system allows therefore multiple transmission paths for spatial division multiplexing. The MU-MIMO system may thus have multiple users served at the same time on the same subcarrier by means of spatial separation. The main challenge in MU-MIMO systems is that not all antennas can cooperate among different user equipments. Therefore, the network transmission unit has to manage inter-user interferences. This is in particular the case in systems such as Distributed Antenna Systems (DAS) or Coordinated Multi-Point transmission/reception (CoMP) systems. In such a system, a Base Station (BS) is equipped with a powerful Base Band Unit (BBU) and is connected to a plurality of Remote Radio Units (RRU) over e.g. high-speed optical fibers. This allows covering large geographical areas. The base band processing as well as radio resource and network management functionalities are performed by the BBU. The RRU are equipped with multiple antennas and perform conversion between Radio Frequency (RF) and digital Intermediate Frequency (IF) signals. In such a DAS system, multiple users may get paired in order to collaborate with each others for transmission.
A MU-MIMO system uses a precoding scheme and a corresponding coding scheme in the network transmission unit for spatial user separation. There are actually two kinds of precoding schemes: Channel Vector Quantization (CVQ) described in “Comparison between MU-MIMO codebook-based channel reporting techniques for LTE downlink” (Philips, 3GPP TSG RAN WG1, 46bis R1-062483, October 2006) and Per User Unitary and Rate Control (PU2RC) described in “Downlink MIMO for EUTRA” (Samsung, 3GPP TSG-RAN WG1, 44 R1-060335, February 2006). In CVQ, the network transmission unit receives a feedback or codebook index from the user equipment that indicates a codebook entry. The codebook is stored both in the network transmission unit and in the user equipments. The codebook is a matrix that comprises quantization channel vectors. The codebook index indicates the quantization channel vector selected by the user equipment. The quantization channel vector selected by the user equipment corresponds to the quantization channel vector of the codebook which is the most representative of the actual used channel estimated by the user equipment. Using the codebook index, the network transmission unit may thus determine in the codebook which quantization channel vector or precoder represents the best the channel of the user and schedule the different users on different subcarriers accordingly. The other precoding scheme PU2RC defines possible precoding non-quantization vectors. In this case, the feedback received by the network transmission unit indicates a preferred precoder from a set of predefined precoders. There is no calculation and therefore there cannot be any optimization based on a feedback from the user equipment. In both existing precoding schemes, once a precoder has been selected, a scheduling scheme is performed by the network transmission unit. The network transmission unit schedules thus the users for transmission according to the combination of quantization channel vectors that gives the best throughput.
Although DAS systems allows increasing network capacity and coverage, the existing precoding schemes and corresponding scheduling schemes are not convenient in such systems for reducing inter-user interferences. This is due to the fact that a plurality of RRU are spread around a given UE that perceives therefore a plurality of different propagation paths. The given UE may thus, in such a case, not detect the signal strengths of some of the RRU when they are under its sensitivity (e.g. when signals are deeply attenuated by the environment) and therefore consider them as equal to zero. In this case, the BS, which does not know that the UE does not receive every signal from every RRU as the quantization channel vector of the corresponding index does not indicate it, performs the scheduling while assuming wrongly that the signal transmitted by all the RRU are actually received and exploited by the UE. Resources are therefore not efficiently or optimally scheduled (i.e. not scheduled in an optimum way).
Today there is no solution to efficiently schedule resources that allow optimizing network resources and thus improving efficiency of such wireless telecommunication systems.
Today there is a need for a scheduling solution that can be easily implemented on the existing communication infrastructures.